The invention relates to testing of integrated circuit devices, particularly to testing of devices indicated as having failed an earlier test.
When integrated circuit devices are being tested on a test floor and certain types of failures are noted, a failure analyst may want to examine the bare die in detail to determine where the failure is occurring.
In the past, dice have been examined by using an emission microscope to look through the top of the die at the lower metal layers and the semiconductor substrate (transistors) to determine where the failure occurred. The die must be electrically connected so that the location of a light emission source in the die (the failure location) can be determined. But as technology has progressed, integrated circuit devices are now manufactured using up to five layers of metal, and it is not possible to look at the lower metal layers and semiconductor substrate through the top of the die. So more elaborate techniques have been developed to look at the back side of the die to examine the lower layers.
It is more difficult to examine the back side when the die is electrically connected since the die is typically mounted to some sort of supportive substrate or mounting plate that is visually opaque. Some companies, for example Hypervision and Hamamatsu, sell an expensive machine for polishing off the substrate that supports the back of the die so the back side of the die can be examined. However, the process takes hours and the die is fragile and easily subject to breakage during testing or polishing.
An easier, faster, more reliable, less expensive method allows the failure analyst to look at the back side of the die without polishing any material away. According to one embodiment of this inventive method, the die is placed into a specially prepared cavity in a BGA or other substrate, held in place by an adhesive tape, wire bonded to the BGA, and encased in epoxy for testing. In the case of packaged devices, the die is first removed from its original substrate (which also removes its bond wires) prior to placement into the specially prepared cavity on the BGA. In the case of a die that is still part of a wafer, the wafer must be diced to separate the die from the wafer.
After the die is electrically connected and encased in epoxy, the adhesive tape is then removed. Thus the back side of the die is exposed while the die is electrically connected, and can be examined while test vectors (or simpler signal combinations) are being applied to the die. An emission microscope from Carl Zeiss or Hypervision can be used for examining the back side, and this microscope allows the failure analyst to very accurately pinpoint the location of the failure.
The procedure has the advantage that the back side of the die is directly attached to an adhesive tape that sits on a firm surface. Thus, the die is held firmly while being bonded to the BGA substrate. Also, when the adhesive tape is removed, the back side is directly accessible.
Preferably the adhesive tape is non-stretchy, and not spongy in a vertical direction (when the tape is lying horizontally). Also, preferably the tape is fairly easy to remove after the epoxy has cured. Preferably the epoxy is hard enough that any deformity during removing the adhesive tape is small enough that no bond wires are broken in the process. A commercial grade low cost mounting resin epoxy from Buehler(copyright) of Lake Bluff, Ill., USA has satisfactory properties.
According to another embodiment, the BGA substrate is not used. Instead, the bare die is placed on a glass slide with its semiconductor substrate side adjacent to the glass, and may be attached to the glass using epoxy, wax, or plastic (for example xe2x80x9cAugerxe2x80x9d). This attaching material should melt in the range of 70-250xc2x0 C., preferably 100-150xc2x0 C., and should be strong enough to hold the die at all testing temperatures. Also, the attaching material should not be compressible when cured. After the die is attached to the glass slide, the glass slide is attached to a probe card so that the upper surface of the die is exposed through a testing hole in the probe card. Then bond pads on the die are bonded directly to bond fingers of the probe card. The die can then be examined at either the front or back side while it is electrically connected to the probe card, which is in turn connected to a tester, and the location of a failure can be determined.